The present invention relates generally to screen printing presses and relates more particularly to an improved carriage drive mechanism for use in a screen printing press.
In a typical screen printing press, ink is applied to a sheet of stock by a squeegee which is carried in reciprocating motion along a printing head by a carriage. Printing occurs as the carriage travels from the front of the head to the rear of the head. Upon reaching the rear of the printing head, the carriage reverses direction and carries the squeegee back to its starting point at the front of the printing head. The path of the carriage from the front to the rear of the printing head is the printing stroke. To produce prints of various lengths, it may be desirable to vary the length of the printing stroke.
Various types of apparatus have been employed in the past for driving the carriage. Such apparatus generally includes an electric motor which transmits motion to the carriage through some type of mechanical linkage.
Various types of systems have been used in the past for adjusting the length of the printing stroke. Electrical sensors have been used to switch on and off the motor which drives the carriage, but electrical stroke control systems have proven unreliable and have additionally been relatively expensive. Pneumatic systems which have been used have also proven unreliable due to problems such as sticking and leakage around air seals.
The most reliable known systems are those wherein the length of the stroke is varied by adjusting the mechanical linkage between the drive motor and the carriage. Typically such an adjustment involves loosening or removing a bolt on a lever arm and replacing and/or retightening the bolt at another location on the lever arm. One mechanism of this type is described in U.S. Pat. No. 3,859,917 to Bubley et al. This type of system, while reliable, had had disadvantages in that adjustment of the stroke requires stopping the press and removing access covers to the linkage inside the machine. The length of the linkage was then changed by shifting the bolt.
Accordingly, it is an object of the present invention to provide a carriage drive mechanism for a screen printing press which provides a convenient and reliable means for altering the length of the printing stroke.